The present invention is related to an apparatus, method, and system for determining a time of arrival, hereinafter abbreviated to TOA, of a transmitted or emitted signal receivable at different locations of known spatial coordinates. More particularly, the invention allows to determine time differences of arrival, hereinafter abbreviated to TDOA. Furthermore, the present invention relates to location-aware or location-based applications built upon wireless networking systems or wireless tag tracking systems.
Newly emerging indoor wireless systems supporting precise and nearly real-time localization and tracking of multiple mobile radio terminals or radio tags require capabilities for accurate measurement of TOA and TDOA of a radio signal receivable at different spatial locations. In particular, wireless local area networks known today have not been designed to sufficiently accommodate these required capabilities and thus often provide insufficient support for location-aware or location-based applications. It would be an advantage if TOA and TDOA measurements could be performed without need for absolute time synchronization between system components located at different spatial positions.
A well-known technique for locating a signal source is based on measuring the difference in time for a signal to travel to a pair of spatially separated receivers of known position. When using a sufficient number of different pairs of receivers ambiguous location solutions can be avoided and the location of the signal source can be found at the corresponding intersection of hyperbolas or hyperboloids. Besides the specific geometric constellation of the receivers and observed signal source as well as the accuracy with which the fixed receiver locations have been measured, the accuracy with which the location of the signal source can be determined depends also critically on the achievable accuracy of the TDOA measurements. Moreover, the timeliness of the obtained location solution is dictated by the delay between time of emission of the signal and time of availability of the location solution. In some cases, it may also be desirable to determine the absolute TOA of the signal at the individual receivers. A plurality of techniques and applications exist that use either radio signals, optical signals, or acoustic signals, and perform TDOA measurements for the determination of the spatial coordinates of the respective sources emitting any such signals.
The paper entitled xe2x80x9cHyperbolic location errors due to insufficient numbers of receivers,xe2x80x9d by John L. Spiesberger, published in Journal of the Acoustical Society of America, 109 (6), June 2001, is related to location techniques based on the difference in travel time of acoustical sources at pairs of widely separated receivers. Therein, the author demonstrates that ambiguous location solutions based on TDOA measurements for two and three spatial dimensions can generally only be avoided by using four and five receivers, respectively. This paper provides a theoretical method to compute the location of an acoustic signal source based on TDOA measurements from up to four independent pairs of receiving stations. However, the paper does not discuss nor disclose any practical solution that could be used for useful TDOA measurements.
U.S. Pat. No. 6,054,950 is related to an ultra wideband precision geolocation system. The system includes N greater than 2 untethered ultra wideband transceivers, hereinafter abbreviated to UWB transceivers, located at fixed positions, an untethered UWB transceiver at the unknown target location (herein also called the unknown location of the signal source), and a processor at the target location. The latter resolves time-of-flight measurement ambiguities of received pulses to determine the geolocation by solving a set of equations according to time-of-flight measurements and surveyed positions of Nxe2x88x921 receivers. To eliminate a clock distribution system, self-synchronizing of pulse timing is achieved by generating a start pulse at one of the untethered transceivers. As an alternate means, a timing source may be provided at the transceivers by a Global Positioning System, hereinafter abbreviated to GPS, or other timing generator, in order to synchronize emissions of their pulses. The system has the disadvantage that it is restricted to situations where both the signal source and the processor estimating the a priori unknown location of the signal source must be located at the same spatial position. Thus, the system is restricted to the known xe2x80x9cmobile-based architecturexe2x80x9d. Also, to eliminate a clock distribution system for time synchronization, the system described in U.S. Pat. No. 6,054,950 has the disadvantage that self-synchronization of pulse timing is achieved in a sequential fashion. An alternate synchronization method as disclosed in U.S. Pat. No. 6,054,950 has the disadvantage that each reference transceiver is required to provide its own GPS-derived absolute timing source.
U.S. Pat. No. 6,028,551 describes a micro-miniature beacon transmit-only geolocation emergency system for personal security, which can operate synergistically with existing or newly designed satellite or ground-based wireless communication networks. The document also discloses a program procedure to calculate the geolocation of the micro-miniature beacon from TDOA measurements at the satellites and initial estimates of the location. The FIGS. 2 and 9 in U.S. Pat. No. 6,028,551 show a configuration or mode of operation of a system architecture that is commonly identified by those knowledgeable in the field as a xe2x80x9cnetwork-centric architecture.xe2x80x9d The document proposes in general terms the use of a traditional and well-known radio-astronomy technique, called auto-correlation, to determine the time difference of signals detected at three or more satellites.
The paper entitled xe2x80x9cPerformance of ultrawideband SSMA using time hopping and M-ary PPM,xe2x80x9d by Fernando Ramirez-Mireles, published in IEEE Journal on Selected Areas in Communications, Vol. 19, No. 6, June 2001, analyzes multiple-access performance in free-space propagation conditions, in terms of the number of users supported by the system for a given bit error rate, the signal-to-noise ratio, the bit transmission rate, and the number of signals in a set of pulse position modulated signals.
In the paper entitled xe2x80x9cIndoor Geolocation using OFDM Signals in HIPERLAN/2 Wireless LANs,xe2x80x9d by Xinrong Li, et al, published in Proceedings of the 11th International Symposium on Personal, Indoor and Mobile Radio Communication (PIMRC 2000), Vol. 2, London, Sep. 18-21, 2000, the authors study and describe new methods to integrate geolocation functionalities into next generation wireless LANs based on OFDM (orthogonal frequency division multiplexed) signals. In particular, the authors propose a method to measure geolocation metrics by exploiting the HIPERLAN/2 MAC frame structure with a focus on geolocation methods for network-based architectures. This paper reports results from computer simulations to show the performance of the investigated geolocation system. The HIPERLAN/2 system is specified to operate in the 5 GHz frequency range and support short-range broadband wireless access within 30 m in typical indoor environments. The authors conclude that their system has the disadvantage of a large mean ranging error of 3 m to 7.5 m, depending on channel conditions. They further conclude that some other timing method is needed to improve the accuracy in real multi-path indoor environments.
The paper entitled xe2x80x9cAn Overview of Wireless Indoor Geolocation Techniques and Systems,xe2x80x9d by Kaveh Pahlavan, et al, published in Proceedings of Mobile and Wireless Communications Networks (MWCN 2000), Paris, France, May 2000, provides an overview of various indoor geolocation systems, including results on predicted performance of such systems. The paper points out that compared to the TOA method, the main advantage of the TDOA method is that knowledge of the transmit time from the transmitting source to be located is not required, while the TOA method does require this information. The paper also states, however, that with TDOA strict time synchronization among all the receivers is required. The paper further shows that available channel bandwidth fundamentally influences the achievable accuracy of indoor geolocation systems. For example, it is shown that an accuracy of less than 1.5 m requires a bandwidth of more than 20 MHz. Thus, present systems with limited bandwidth and limited range of some 30 m are limited to relative position errors of no less than 5%, and the relative position error increases significantly with decreasing range.
Therefore, in view on the mentioned documents, a need exists for a practical solution for precise and timely determination of TOA at individual receiving stations and determination of TDOA at pairs of receiving stations, where the solution is not limited or restricted by:
a strictly xe2x80x9cmobile-based architecturexe2x80x9d,
sequential time synchronization of communicating stations,
a need for reference transceivers with on-board GPS-derived absolute timing source,
large absolute and relative mean ranging errors in real multi-path environments,
a need for strict distributed time synchronization among all the receivers.
A feature of the present invention is to overcome the disadvantages and the limitations of the prior art.
Another feature of the present invention is to determine a time of arrival at one location or several locations, of a signal transmitted from an a priori unknown location at an arbitrary time.
Still another feature of the present invention is to determine a time difference of arrival at two or more locations, of a signal transmitted from an a priori unknown location at an arbitrary time.
The present invention provides a solution for determining a time of arrival tn, abbreviated to TOA, and if desirable the absolute TOA, of one or a plurality of signals emitted by one or a plurality of transmitters, e.g. terminals or radio tags, and receivable at different locations of known spatial coordinates. In particular, the solution is based on measuring and comparing the total signal transfer time (TSTT) of signals, in such a way that explicit knowledge of absolute time or absolute time synchronization between system components located at different spatial positions is not required.
In accordance with the present invention, there is provided a system, method, and apparatus for determining the time of arrival tn of a transmitted signal {S}. The transmitted signal {S} is receivable at different receiving locations L, the signal {S} being transmittable by a first transmitter. In one aspect of the present invention the apparatus comprises at least two transponder units, each comprising a first receiver for receiving the transmitted signal {S}, a time-processing unit coupled to the first receiver for generating, in response to receipt of the transmitted signal {S} by the first receiver, a first time-count value xcex4n, and for scheduling transmission of a time-processed signal {{tilde over (S)}n}, which is based on the transmitted signal {S}, in dependence on the time-count value (xcex4n) and a transmitter for transmitting the time-processed signal {{tilde over (S)}n} from a transmission location Lxe2x80x2n. The apparatus further comprises an evaluation unit. The evaluation unit comprises a second receiver for receiving the time-processed signal {{tilde over (S)}n} after a determinable time-delay value xcfx84n, a timer unit coupled to the second receiver for generating, in response to receipt of the time-processed signal {{tilde over (S)}n} by the second receiver, a second time-count value xcex94Tn, a controller for generating a time reference T0 for the timer unit, and a processing unit for deriving the time of arrival tn based on the time reference T0 from the first time-count value xcex4n, the time-delay value xcfx84n, and the second time-count value xcex94Tn.
It can be advantageous if the time reference T0 is an absolute time reference, because then also the time of arrival tn can be determined as an absolute time of arrival.
From at least two of the derived time of arrival tn or absolute time of arrival a time difference of arrival (TDOA) can easily be calculated by calculating the difference. The transmitted signal {S} can comprise a first transmitter identification information. This is advantageous when multiple first transmitters are used, because then each first transmitter can be uniquely identified.
The time-processed signal {{tilde over (S)}n} can comprise a second transmitter identification information, the first transmitter identification information, and the first time-count value xcex4n. Then the time-processed signal {{tilde over (S)}n} is easily distinguishable and the contained information can be processed immediately.
It is advantageous when the determinable time-delay value xcfx84n is derivable for a given geometrical arrangement of the transponder units, because then the determinable time-delay value xcfx84n is determined or measured only once instead of several times and therefore a faster processing can be achieved.
A network-centric architecture is assumed with the accompanying advantage that a mobile station does not need to extract location metrics from received signals transmitted by a network of stations and also does not require a processor to compute an estimate of its spatial location. Rather, a transmitter of a mobile radio terminal or radio tag simply emits, independent of other possible radio terminals or radio tags, a suitably identifiable signal that is receivable at different receiving locations L. Basically, only the network-side of the system includes a processor or processing unit to provide estimates of the spatial location of an observed mobile signal source. If desired, a mobile signal source may in addition be equipped with a suitable receiver to become a mobile transceiver or transponder unit that can also receive estimates of its spatial location as computed and transmitted by the network-side.
In general, the system comprises a set of first transmitters where the antenna feed point of each first transmitter is positioned at an a priori unknown spatial location. Each transmitted signal {S}, also referred to as first signal {S}, emitted in an independent and uncoordinated fashion according to the principles of code division multiple access (CDMA), carries at least information for the unique identification of the particular first transmitter, i.e. the first transmitter identification information. The system further includes at least two transponder units, each unit comprising the first receiver capable of receiving the first signal {S} by means of an antenna having a feed point located at an a priori known spatial location, and the second transmitter emitting the time-processed signal {{tilde over (S)}n}, also referred to as second signal {{tilde over (S)}n}, derived from the received first signal {S}. A link between any first transmitter and any first receiver will hereafter be referred to as a connection on xe2x80x9cLink 1xe2x80x9d. Connections on Link 1 make use of the known principles of CDMA with the added advantage that these connections can operate in parallel rather than sequentially, yielding a time saving advantage.
It is an advantage of the invention to make use of ultra wideband radio technology (UWB-RT) in combination with code division multiple access (CDMA), i.e. using ultra wideband radio signals modulated according to the principles of CDMA, to achieve parallel and timely measurement and collection of the total signal transfer times (TSTTs).
It is a further advantage that a signal""s TSTT can be measured without need for strict distributed time synchronization among all the receivers. Moreover, the TSTT can be measured based on only a single point of synchronization and without need to know absolute time information.
With the present invention the implementation of location-aware as well as location-based applications for wireless networking systems or object-tracking wireless systems can be enabled. This can also be used for wireless systems operating in an indoor environment.
Another advantage of the present invention is that sufficient information to the processing unit at an arbitrary or a priori unknown location can be delivered, such that it can compute an estimate of the a priori unknown location of the signal source or first transmitter.
The second signals {{tilde over (S)}n} are also emitted in an independent and uncoordinated fashion according to the principles of CDMA by a set of second transmitters whose antennas may or may not be identical with the respective first receiver""s antennas. The second signals {{tilde over (S)}n}are 20 received by a bank of second receivers that make use of either the same antenna or individual antennas and whose spatial location may or may not be known. A link between any second transmitter and any second receiver will hereafter be referred to as a connection on xe2x80x9cLink 2xe2x80x9d. Connections on Link 2 make use of the known principles of CDMA with the added advantage that these connections can operate in parallel rather than sequentially, yielding a time saving advantage.
The second receiver, or possibly bank of second receivers, is connected to a common controller. The controller provides in a first step appropriate reference time indications that can be used to terminate a measurement cycle for a particular mobile radio terminal or radio tag. In a second step, the controller delivers to the processing unit a set of third signals {Ŝn} for each mobile radio terminal or radio tag that are derived from corresponding second signals {{tilde over (S)}n}. The link between the controller and the processing unit will hereafter be referred to as a connection on xe2x80x9cLink 3xe2x80x9d. The processing unit can make use of the received set of third signals {Ŝn} to estimate the spatial location of the first signal source; generally, the processor provides this location estimate for further use to other entities of the system or to a suitable application program.
It is an advantage of the present invention that the first and second receivers are able to resolve the arrival time instant of a first signal with high resolution provided that all first transmitters, all first receivers, all second transmitters, and all second receivers make use of UWB-RT. For example, U.S. Pat. No. 6,054,950 and references mention that an arrival time resolution of much better than 100 ps is achievable with UWB-RT, enabling spatial resolution on the order of centimeters. Thus the transition time of the second signal {{tilde over (S)}n} on any Link 2 can be accurately determined in advance, for example during a calibration step when the system components are installed in a particular environment. Upon detection of one first signal {S}, each of the second receivers starts a first time counter at a non-zero initial count corresponding to the scheduled relative transmission time of the derived second signal. The scheduled relative transmission time, in this document referred to as first time-count value xcex4n or xe2x80x9cDelay 1xe2x80x9d, corresponds to the time of emission of the second signal {{tilde over (S)}n}with the timing phase referred to the second transmitter""s antenna feed point. In other words, the first time-count value xcex4n is the total delay between transponder""s receive and transmit antenna feed point. Each second transmitter prepares and schedules its second signal for transmission over Link 2 independently and each second signal {{tilde over (S)}n} includes at least i) information for unique identification of the first receiver, ii) information on the identity of the first signal source, and iii) xe2x80x9cDelay 1.xe2x80x9d Depending on the designer""s choice, the specific known location of the second receiver and/or the previously determined second signal transition time on Link 2 may also be directly included in the second signal {{tilde over (S)}n}. Upon detection of a corresponding second signal {{tilde over (S)}n}, each of the second receivers starts a second time counter at initial count zero, stopping it only at its final second time count when the common controller issues a corresponding stop-signal, simultaneously to all second time counters associated with a particular identified mobile radio terminal or radio tag. This single reference time event initiates the end of an active TOA or TDOA measurement cycle for a particular identified mobile radio terminal or radio tag. The controller generates for each identified mobile radio terminal or radio tag a separate single reference time event at the end of an active TOA or TDOA measurement cycle.
The controller then prepares for each second receiver an appropriate third signal {Ŝn} for transfer over Link 3, this third signal {Ŝn} including at least i) information on the identity of the first signal source, ii) information on the identity of the second receiver, iii) Delay 1, iv) the previously determined time-delay value xcfx84n on Link 2 that can be regarded as second signal transition time, and v) the final second time count or second time-count value xcex94Tn, hereafter also called xe2x80x9cDelay 2.xe2x80x9d Alternatively, the third signal {Ŝn} may also include the specific known location of the second receiver and/or a total sum consisting of three terms, i.e., Delay 1, the previously determined time-delay value xcfx84n on Link 2, and Delay 2. Thus, the system yields the advantage of only requiring a single reference time to be made available in the single controller controlling the second receivers. The total time interval between the time instant of a first signal arriving at a first receiver and the time instant when the second time count is stopped is the sum of only three known individual time intervals, namely, Delay 1, transfer time on Link 2, and Delay 2. Thus, a major advantage of the present invention is given by the fact that the sought TOA or TDOA measurements can be obtained by the processing unit from computing such corresponding total time intervals or from computing differences thereof, respectively. Strict and absolute time synchronization among all the first and second receivers is not required and independent clock signal generators with certain specified frequency and stability are sufficient. The use of particularly wide channel bandwidths, e.g., as obtained when using system components based on UWB-RT, achieves precise TOA and TDOA measurements and reduced sensitivity to multi-path propagation.
The present invention provides furthermore the opportunity to determine the TOA of first signals {S} at first receivers. This can be achieved provided that the stop-signal issued by the controller to all second time counters can be related to an absolute time reference. This could be achieved, for example, by making use of the network time reference or the time information obtained from a separate time reference receiver that is connected to the controller, such as a GPS receiver. The set of third signals {Ŝn} is transferred to the processing unit over the connection previously called Link 3, where the latter can be implemented by means of wires or by means of other transmission media. Generally, the set of third signals {Ŝn} received by the processing unit enables the latter to provide an estimate of the spatial location of the first signal source identified by the set of third signals {Ŝn}.
The present invention provides a practical solution for precise and timely determination of TOA at individual receiving stations and determination of TDOA at pairs of receiving stations, where knowledge of absolute time or absolute time synchronization between system components located at different spatial positions is not required to determine a particular signal""s path propagation time within specified accuracy. The system can be applied to applications that require an up-to-date and accurate estimate of the a priori unknown spatial location of the source emitting a receivable signal.
The system avoids certain disadvantages found in systems based on a xe2x80x9cmobile-based architecture,xe2x80x9d systems using sequential synchronization of communicating stations, or systems requiring strict time synchronization among all the receivers. Rather, the proposed solution provides advantages and features such as those derivable from systems based on a xe2x80x9cnetwork-based architecture,xe2x80x9d communication systems using multiple-access technology (e.g., CDMA), systems without need for strict time synchronization among all receivers, systems operating with independent clock signal generators of some specified frequency and stability. All these benefits can be enhanced when the underlying communication devices make suitable use of UWB-RT.
In summary, the present invention provides the following advantages and features:
TOA and TDOA determination require only a single point of synchronization without need to know absolute time information,
not restricted to xe2x80x9cnetwork-based architecture,xe2x80x9d
use of multiple-access technology (e.g., CDMA) to achieve parallel and therefore timely generation and collection of determination results,
no need for strict distributed time synchronization among all the receivers; independent clock signal generators with certain specified frequency and stability are sufficient,
use of particularly wide channel bandwidths (as provided by UWB-RT) to achieve precise TOA and TDOA determination with reduced sensitivity to multi-path propagation.